DRTVWR-489-emoji: As part of the work to get macOS version of the Viewer...

DRTVWR-489-emoji: As part of the work to get macOS version of the Viewer working, the flag was introduced to warn (and therefore error out) when a virtual override was not marked with the 'override' keyword. Fixing this up involved a large number of changes and this commit represents just those changes - nothing specially from the DRTVWR-489 viewer